Characterization of an ambident electrophile: the gas phase reactivity of the methoxymethyl cation

Gas-phase electrophilic addition promoted by CH(3)S(+)=CH(2) ions on aromatic systems

The gas-phase methylenation reaction between CH(3)S(+)=CH(2) and alkylbenzenes, aniline, phenol and alkyl phenyl ethers, which yields [M + CH](+) and CH(3)SH, has been studied by Fourier transform ion cyclotron resonance (FT-ICR) techniques and computational chemistry at the DFT level. The methylthiomethyl cation is less reactive than methoxymethyl and, unlike the latter, is unreactive toward benzene. The calculations suggest that reaction with toluene should proceed primarily by addition at the para and ortho positions resulting in a benzyl-type ion. Reaction with aniline-2,3,4,5,6-d(5) reveals that elimination of CH(3)SD is kinetically favored by a factor of 5 over elimination of CH(3)SH. Experiments with C(6)H(6)ND(2) and theoretical calculations suggest that methylenation at the nitrogen atom is energetically favorable and likely, but the observed results may reflect some H/D scrambling, which occurs after attack at a ring position. By comparison, reaction with phenol-2,3,4,5,6-d(5) reveals that methylenation followed by elimination of CH(3)SD is kinetically favored by a factor of 3.8 over elimination of CH(3)SH. For phenol, the theoretical calculations suggest that attack by CH(3)S(+)=CH(2) at the para or ortho position is the only low-energy pathway for methylenation. However, a low-energy pathway for hydrogen scrambling is predicted by the calculations originating from the exit complex, [CH(3)SH(...) CH(2)=C(6)H(4)=OH](+), of reaction at a ring position.

Quadrupole ion trap studies of fundamental organic reactions

Over the past several decades, quadrupole ion traps have become important instruments in the study of gas phase ion/molecule reactions. As a part of this work, they have been employed in a number of studies focused on fundamental organic reaction mechanisms. In this review, the general features and limitations of quadrupole ion traps are discussed followed by a description of representative ion trap studies of organic reaction mechanisms such as substitution, elimination, and acyl transfer reactions.

Reactions of CH3OCH2+ with hydrocarbons and O, N, and S compounds: applications for chemical ionization in selected ion flow tube studies

We report the results of a flowing afterglow ion source-selected ion flow tube study (FA-SIFT) of the reactions of the methoxymethyl cation, CH3OCH2+. Rate coefficients and product branching ratios are reported for twenty nine reagent molecules including those that constitute the major ingredients of air, the hydrocarbons CH4, C2H6, C3H8, n-C4H10, C2H2, C2H4, C3H4 (allene and propyne), C6H6, and the S-containing molecules H2S, CH3SH, C2H5SH, (CH3)2SH, and (C2H5)2SH. In addition, we examined the reactions with the N-containing molecules NH3, CH3NH2, (CH3)2NH, (CH3)3N, pyrrole, pyridine as well as CH3COCH3. The results can be summarized under three general reaction types: Reaction at the CH3 carbon, reaction at the CH2 carbon, and association. The results also indicate that the methoxymethyl cation can be used as a chemical ionization source for the detection of trace levels of S-containing compounds in saturated hydrocarbons.

Gas-phase SN2 reactivity of dicoordinated borinium cations using pentaquadrupole mass spectrometry

We report gas phase ion/molecule reactions between dialkoxyborinium cations (RO-B+-OR) and small organic amides, such as N,N-dimethylformamide and N,N-diethylpropionamide. Besides direct addition at boron, the results show efficient SN2 methyl transfer from the borinium ion to the amide. Isotopic labeling and collision-induced dissociation (CID) of the methyl transfer products demonstrate O-methylation of the amide. Methyl substitution at the alpha-carbon of the amide affects the degree of alkylation and adduct formation. Direct proton abstraction via beta-elimination is a major competitive reaction for substituents other than methyl. Ab initio calculations at the B3LYP/6-31G(d) level indicate that SN2 transmethylation is highly exothermic with O-methylation favored over N-methylation by 14.8 kcal/mol.

Mass spectrometric studies of organic ion/molecule reactions

In the past 25 years, a tremendous amount of work has been published on the ion/molecule reactions of organic species. This review provides an overview of the areas where gas phase ion chemistry has made a contribution to our understanding of fundamental organic reaction processes. It is clear that the gas phase work can provide insights into subtle features of reaction mechanisms that could not be addressed by conventional condensed phase methods. The study of ion/molecule reactions has already had a major impact on the way that organic chemists think about reaction mechanisms and interpret substituent effects. Moreover, it has heightened our awareness of the importance of solvation effects and how they can alter not only absolute rates but also relative rates, leading in some cases to complete reversals in reactivity patterns. A large body of work could not be included in this review due to space limitations. For example, the study of thermochemistry in the gas phase (i.e., acidities, basicities, bond strengths, binding energies, etc.) has provided a wealth of data that has been exceptionally useful in interpreting organic reaction mechanisms. This has spilled over into the study of organometallic systems, and several groups are making major headway in using mass spectrometry to probe the stability and reactivity of transition metal species. Finally, work involving chemical ionization has provided abundant information on gas phase reaction mechanisms. The future appears to be very promising for the study of gas phase organic reaction mechanisms. In particular, the emergence of new ionization techniques and more powerful mass analyzers will allow chemists to explore a wider range of species. Although still at an early stage, the gas phase study of biochemical transformations offers great promise and has been facilitated by electrospray and matrix assisted laser desorption ionization methods. In addition, these techniques provide a means for introducing important, metal-centered catalytic species into the gas phase and exploring the details of their reactivity. Finally, mass spectrometry continues to play a major role in the study of atmospheric ion chemistry and is providing important kinetic as well as mechanistic data.